Due to its wide distribution and ready access, the telephone link has become a major communication medium for a variety of telecommunication and information processing applications. Because a conventional copper link has a very limited bandwidth (typically on the order of 3.3 KHz), it is usually necessary to compress the data prior to transmission. Consequently, considerable time and effort have been spent in both communications and signal processing technologies to produce compression mechanisms that operate so as optimize the quality of the recovered signal at the receive end of the channel.
One compression mechanism that has proven particularly useful for digital image signal processing is adaptive block transform compression, as described in the U.S. Pat. Nos. 4,774,574 (hereinafter denoted '574) and 4,780,761 (hereinafter denoted '761) to Daly et al, each assigned to the assignee of the present application and the disclosures of which are incorporated herein. In accordance with these patented schemes, (DCT) transform coefficients are normalized in accordance with a model of the human visual system. Since the human system model reveals aspects of decreased visual sensitivity, then by normalizing the transform coefficients so that the contribution of such aspects is substantially reduced, a further enhancement in the compression ratio can be realized. Although the patents describe the application of the adaptive transform compression mechanism to monochromatic images, they indicate that it may also be applied to the luminance component of color images. However, the chrominance components are still processed with constant normalization, since the resulting processing gain does not justify the added complexity.
Because color imagery data contains additional components and is inherently more complex than monochromatic data, it is customarily subsampled (filtered and decimated) in order to reduce the bandwidth prior to compression. For conventional color imagery data processing, low pass filtering and subsampling is carried out globally, namely over the entire image. However, global filtering is not possible in the above-referenced patented systems, since data is processed in block format. Consequently, filtering and subsampling of the block-formatted data would require an extended addressing scheme, placing a substantial memory burden on the signal processor and reducing signal processing speed.